1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which is called In-Plane Switching Mode.
2. Description of the Related Art
A liquid crystal display device of the type which is called In-Plane Switching Mode has a construction in which a pixel electrode and a counter electrode are formed in each liquid-crystal-side pixel area of either one of the substrates disposed in opposition to each other with a liquid crystal interposed therebetween, so that the optical transmissivity of the liquid crystal is controlled by a component which is contained in an electric field generated between the pixel electrode and the counter electrode and which is nearly parallel to the substrates.
It has been known that one type of such a liquid crystal display device has a construction in which its pixel electrode and its counter electrode are respectively formed in different layers with an insulating film interposed there between, and either one of the pixel and counter electrodes is formed as a transparent electrode formed in nearly the whole of each pixel area, while the other is formed as plural stripe-shaped electrodes which are disposed in nearly the whole of each pixel are a in such a manner as to be extended in one direct ion and to be juxtaposed in a direction traverse to the one direction.
This kind of art is described in detail in, for example, K. Tarumi, M. Bremer, and B. Shuler, IEICE TRANS. ELECTRON., VOL. E79-C No. 8, pp. 1035-1039, AUGUST 1996.
Incidentally, a so-called active matrix system is applied to such a liquid crystal display device; for example, each of its pixel areas is surrounded by adjacent ones of gate signal lines disposed to be extended in the x direction and to be juxtaposed in they direction and by adjacent ones of drain signal lines disposed to be extended in they direction and to be juxtaposed in the x direction, and each of the pixel areas is provided with a switching element to be driven by the supply of a scanning signal from one of the adjacent gate signal lines, and a pixel electrode to which a video signal is to be supplied from one of the adjacent drain signal lines via this switching element.
However, it has been known that such a liquid crystal display device suffer light leaks which occur by its liquid crystal being driven by electric fields which are generated between its drain signal lines or its gate signal lines and electrodes disposed adjacently thereto.
As a counter measure against this phenomenon, there is a method which blocks light due to such light leaks by means of a black matrix formed on the opposite transparent substrate. However, in this method, the width of the black matrix needs to be ensured to some extent, so that a disadvantage such as a reduction in the aperture ratio of each pixel can not be avoided. Accordingly, a solution using another method has been desired.
A thin film transistor which is a switch element is made of a stacked structure in which a gate electrode connected to a gate signal, a gate insulating film, a semiconductor layer, a drain electrode connected to a drain signal line, and a source electrode connected to a pixel electrode are stacked in that order on the substrate. However, it has been pointed out that electric charge is easily irregularly charged on the side of the surface of the semiconductor layer on which the individual electrodes are formed (this phenomenon is called a back channel), so that non-uniformity occurs in the characteristics of individual thin film transistors.
The invention has been made in view of the above-described problems, and provides a liquid crystal display device in which light leaks due to the influence of signal lines are prevented.
The invention also provides a liquid crystal display device provided with thin film transistors having stable characteristics in which no back channels occur.
Representative aspects of the invention disclosed in the present application will be described below in brief.
A liquid crystal display device includes, for example, in each pixel area on a liquid-crystal-side surface of one of substrates disposed in opposition to each other with a liquid crystal inter posed there between, a switching element to be driven by supply of a scanning signal from a gate signal line, a pixel electrode to be supplied with a video signal from a drain signal line via the switching element, and a counter electrode which causes an electric field to be generated between the counter electrode and the pixel electrode. The counter electrode is formed in a layer overlying the pixel electrode with an insulating film interposed between the counter electrode and the pixel electrode, and a conductive layer is formed in the same layer as the counter electrode and over a signal line which is at least one of the drain signal line and the gate signal line which lie in a layer underlying the insulating film, the conductive layer being superposed on the signal line.
According to the liquid crystal display device constructed in this manner, the conductive layer can have the function of a light shielding film, and can prevent light leaks due to the influence of signal lines.
In addition, since this light shielding film is formed on a substrate on which the signal lines are formed, the light shielding film can be formed to have a width sufficient to prevent the light leaks, whereby the width can be set to be comparatively small.
In the case where the conductive layer is formed to cover the gate signal line, the conductive layer is also formed to cover the thin film transistor for med to be superposed on the gate signal line (with the insulating film being interposed between the conductive layer and the thin film transistor).
Accordingly, if electric charge is present in the insulating film on the thin film transistor, the distribution of the electric charge can be stabilized by the conductive layer, whereby it is possible to obtain stable characteristics which do not allow back channels to occur in the thin film transistor.